The present invention relates to a hydrometallurgical process for the recovery of valuable components, especially gold, silver and platinum, as well as selenium, tellurium, copper, nickel, etc., from the anode slime produced in the copper electrolysis, copper having preferably been removed from the slime in advance. The anode slimes produced in the electrolytic refining of lead, etc., as well as scraps or metallurgical intermediates bearing the above components, can also be treated according to the invention in order to separate silver from them. For the sake of clarity, only the treatment of anode slime from the copper electrolysis is discussed below. The anode slime produced in the refining of copper by electrolysis contains that part of the anode which is insoluble in electrolysis and contains varying quantities of copper, nickel, silver, gold, selenium, tellurium, arsenic, sulfur, lead, silicates, etc., depending on the anode. The main object in treating anode slimes has been to separate the valuable metals, but the separation of byproducts such as selenium and tellurium is usually also profitable.
Owing to the facts that the number of components concerned is great and that in different refineries the compositions of the slime and the local conditions vary greatly, a very great number of different processes are used for the treatment of anode slimes. Some characteristics common to them can be distinguished, however, as is proven in the discussion below.
In the treatment of anode slime, copper and nickel are usually removed first and selenium and tellurium thereafter. Dore metal, which is a silver-gold mixture bearing small quantities of copper and the platinum and palladium present in the slime, is obtained by smelting from the slime purified of the above components. The dore metal is refined by electrolysis, whereby pure silver is deposited on the cathode. Anodes for the electrolytic refining of gold are cast from the anode slime produced in the silver electrolysis; in the gold electrolysis, pure gold is obtained on the cathode and the Pt and Pd dissolved in the electrolyte can be separated from it.
Copper is usually removed by slurrying the slime collected from the copper electrolysis tanks in dilute sulfuric acid, e.g., the cycled solution of the copper electrolysis, and by blowing air through the slurry, whereby the metallic copper dissolves as a sulfate. Some other methods for removing the copper are the oxidizing and the sulfating roasting, whereby copper is obtained in a form soluble in mild acid or water.
If considerable quantities of nickel (10-50%) are present in the anode slime, the nickel can be removed by sulfuric acid roasting within the temperature range 180.degree.-300.degree. C after the separation of copper. The nickel, which is present in the slime as an oxide, converts into a sulfate and is dissolved in water.
The most important methods for recovering selenium are:
1. Sulfating roasting. A slime devoid of Cu and Ni is mixed with sulfuric acid and roasted at a high temperature (500.degree.-600.degree. C), whereby SeO.sub.2 evaporates. The SeO.sub.2 is recovered from the discharge gases into mild sulfuric acid. Raw selenium is produced from this solution by reducing it by means of SO.sub.2.
2. Soda roasting. A mixture of anode slime and soda is roasted at a temperature of 350.degree.-450.degree. C and simultaneously blasted with air. Se and Te oxidize into Na.sub.2 SeO.sub.3, Na.sub.2 SeO.sub.4 and Na.sub.2 TeO.sub.3. These are dissolved in water, from which they can be separated by different methods.
3. Soda smelting. A slime devoid of Cu and Ni is smelted together with soda. Air is blasted into the molten batch and it oxidizes and evaporates part of the Se and Te, which are recovered in the wash of the gases. The slag bearing Se and Te is leached in sulfuric acid or the acid wash solution of the gases. When the solution is neutralized (pH = 5-6), TeO.sub.2 precipitates and the selenium is reduced into elemental form by means of SO.sub.2.
The recovery of tellurium is always based on bringing it into a water soluble state and precipitating it as TeO.sub.2, from which elemental Te can be produced by electrolysis, for example. Te can be caused to dissolve from anode slime either in connection with soda roasting or soda smelting or by boiling the slime together with a base after an oxidizing or a sulfating roasting.
In principle the object in refining anode slime is that the various components to be recovered are completely separated, in a pure form, each at its own stage, before the dore smelting, which is usually the most expensive and complicated stage of the process. The first stage in a dore furnace is the oxidation of lead, nickel and copper by means of air and the compounding of the same with silicate slag. When the slag is removed and the oxidation is continued, the As and Sb are evaporated as oxides. Se and Te are separated by means of soda slag and a niter oxidizer. The metal is cast into anodes for the Ag refining electrolysis.